The present invention relates to fiber optic telecommunication cables and static- wires used with overhead high-voltage power transmission lines.
With the advent of fiber optics, high-volume communication no longer requires the use of bulky copper cables. For example, a single glass fiber having a coated diameter of only 250 micrometers can transmit information at a rate of 565 million bits (about 35,000 typed pages) per second. The "all-dielectric" (non-conducting) attribute of fiber optic cables permits their use in applications unsuitable for metal cables, such as along existing utility power transmission rights of way, since the problems associated with induced voltages are avoided.
Although seemingly fragile, the glass fibers are individually very strong; short-length fibers have shown tensile strengths between 400 KPSI and 800 KPSI at elongations ranging from 4% to 8%. However, telecommunication grade fibers are proof-tested over their entire length only at stresses from 50 KPSI to 100 KPSI. Thus, glass defects that may cause fracture at larger stresses will not be detected and removed. Furthermore, long-term stress even below the short-term proof-test level can cause fatigue failure due to the slow progress of flaws in an otherwise acceptable fiber. Since fiber cables are subjected to various tensile stresses during normal installation and use, a cable design which minimizes the transfer of these loads to the fibers will have increased reliability.
Outdoor, above-ground installations subject cables to a wide variety of rigors. During installation, an overhead cable is passed through a series of installation blocks under relatively low stringing tension and then raised to the design tension of the line: typically in the range of 1,000 to 5,000 lbs for a 500-foot span. The overhead cables called static wires typically have rated breaking strengths in the range of 10,000 lbs to 30,000 lbs. In-service tension can increase to 60% of the rated strength as a result of severe weather conditions such as ice loading; under extraordinary conditions, the tension can increase to as much as 95% of the rated strength. The cable must withstand the environmental temperature range of -40.degree. C. to +70.degree. C. and the resulting cyclical expansion/contraction of the cable should be isolated from the fibers to avoid fatigue failures. The fibers also must be protected from the elements, especially water, and from hydrogen because it is well-known that these can cause increased attenuation in silica glass fibers. Thus, besides being water-tight and corrosion-resistant, the cable materials should allow any hydrogen generated to diffuse to the atmosphere.
A typical overhead installation is that for high-voltage power transmission via conductors suspended from a series of towers or pylons. For efficiency, a load-balanced delta connection may be used having one conductor for each of the three electrical phases. Whether from a lightning strike or other electrical disturbance, one or more of the phase conductors is occasionally unable to carry its share of the power. To avoid a total loss of the transmission system in that case, auxiliary conductors called static wires are often provided to carry the fault currents. The static wires are normally suspended above the phase conductors from the same transmission towers and thus are also exposed to lightning surge currents. As already observed, it is advantageous to carry the fiber cables along existing utility rights of way. However, restrictions on use may prevent simply suspending another cable from the transmission towers. Accordingly, an optical cable that can also function as a static wire provides the significant benefit of allowing the use of existing rights of way for telecommunication.
A fiber optic cable that can be used as a static wire is disclosed in U.S. Pat. Nos. 4,416,508 and 4,491,387. Several embodiments of the cable include one or more fibers enclosed in a tube which is then inserted in the axial bore of a metal member that can take various shapes. The metal member is then wrapped with metal wires which provide the cable's strength. The cables allow some undefined degree of relative movement of the cable elements and the relationship between the lengths of the fibers, tubes, and metal members is uncontrolled. Another optical cable that can be used as a static wire is disclosed in U.S. Pat. No. 4,514,058. One embodiment of the cable includes a central slotted metal member with optical fibers or electrical conductors secured in the slots and then wrapped with metal wires. The very strong central support and the strong wires together resist tensile and radial loads applied to the cable.
Another fiber cable design that can be employed as a static wire is an aluminum pipe formed around a dielectric core tube containing the optical fibers. The longitudinal seam of the pipe may be welded to provide mechanical and environmental protection for the core and the pipe may be wrapped with aluminum-clad steel wires to provide the necessary tensile strength. Use of aluminum provides the high conductivity necessary for the cable's function as a static wire and as an occasional lightning target. However, this approach relies mainly on the rigidity and elasticity of the pipe's wall for resisting the radial force applied when the cable is pulled around a bend or sheave wheel. This force, also known as "sidewall pressure", can permanently flatten the pipe if the elastic limit of the pipe material is exceeded. Since radial forces of 1,000 to 5,000 lb/ft can be expected during typical cable installations, a bend radius of less than 200 to 300 times the pipe diameter results in permanent deformation, even for high strength aluminum. Since a typical bend radius for static wire is 12 inches, aluminum pipes having diameters of 0.3 to 0.4 inches are permanently flattened during normal installation as static wires. This deformation can result in excessive attenuation in the optical fibers due to pressure exerted on the core by the pipe.
Several additional fiber cable designs are disclosed in U.S. Pat. Nos. 3,955,878, No. 4,388,800, No. 4,389,088 and No. 4,491,386 which are directed to submarine installations. In general, the cables disclosed attempt to protect the fibers from tensile stresses by simply twisting them into helices thus increasing their lengths relative to the cable lengths. Single fibers are laid directly into channels in the cable core so that when the cables are stretched, the extra length of the fibers prevents transmission of the cable elongation to the fibers.